org.iets3.core.expr.typetags.units.quantity: QuantityReference and UnitReference Scope implementation not consistent

[arimer]

The way how Quantities and Units should be referenced is not consistent at the moment.

• For QuantityReference there is currently no scope impl. at all. Meaning we rely on the model scope
• For UnitReference a scoping is provided. It enforces that the "context node" where there reference is defined must implement IVisibleElementProvider

I think both should share a common logic, otherwise you end up in a situation, where you mind see Quantities of a Library_A but not the units which might be re-exported by Library_A.

[alexanderpann]

Can we close the ticket if we don't indent to fix it?